1. Field of the Invention
The present invention relates to a method and apparatus for correcting the direction of a deviated borehole that has gone off course during a normal drilling operation. The present invention provides more particularly a self-aligning sleeve with two elliptically-shaped protrusion means for guiding a drill bit in the correct direction after the borehole has deviated from the intended course.
2. General Background
The direction a borehole takes during drilling is influenced by various factors which cause the borehole to deviate from its intended course. These factors include the configuration of the formation through which the borehole is being drilled, the angle of the drill bit and the weight applied to the drill string assembly during the drilling process.
Many devices have been patented and used commercially either to limit the deviation of the borehole or to correct the direction of the deviated borehole. Drill collars and stabilizers are examples of recognized methods of limiting the deviation of a borehole.
Correcting deviation in a borehole may be accomplished by the following devices and techniques which guide and drill pipe and drill bit into the desired direction: a whip stock, a mud motor, a bent sub placed above a downhole motor that is used to drive the drill bit, stabilizers attached to the lower drill string member near the drill bit, and eccentric sleeves.
In U.S. Pat. No. 4,270,619 issued to Base entitled "Downhole Stabilizing Tool with Actuator Assembly and Method for Using Same" teaches the use of a tool that is an independent member that connects into a drill string assembly. The tool has stabilization pads that move radially for selected engagement with the borehole.
U.S. Pat. No. 4,220,213 issued to Hamilton entitled "Method and Apparatus for Self Orienting a Drtill String While Drilling a Well Bore" teaches an eccentric sleeve with a projection means that extends radially beyond the diameter of the drill bit in order to develop as much rigidity as possible in the lower drill string assembly near the drill bit.
U.S. Pat. No. 4,076,084 issued to Tighe entitled "Oriented Drilling Tool" is a sleeve composed of three tubular members inserted into one another. The innermost member is drilled offcenter to make an eccentric sleeve.
U.S. Pat. No. 4,108,256 issued to Moore entitled "Sliding Stabilizer Assembly" teaches a sliding sleeve with helical vanes attached to the outer surface for use in drilling horizontal boreholes.
Amongst other things the aforementioned patents differ from the preferred embodiment of the present invention in three aspects. First, the patents teach an apparatus that rotates coaxially with the drill string assembly, while the present invention remains stationary as the drill string assembly rotates through its hollow center.
Second, the patented tools are designed to remain on the drill string assembly during normal drilling operations, while the preferred embodiment of the present invention is connected to the drill string assembly on an as needed basis. Thus, since the present invention is not in continuous use it is not as susceptible to damage and wear as the patented invention, thus prolonging the life of the self-aligning sleeve and saving rig time that would have to be spent in replacing damaged tools.
Third, the patented tools are designed to connect to the drill string assembly near or adjacent to the drill bit. The additional weight on the drill bit created by the proximity of the patented tools to the drill bit causes the bit to deflect from its intended course.